Method of recognizing the presence of hydrocarbons and associated fluids in reservoir rocks below the surface of the earth

ABSTRACT

This invention relates to methods of recognizing the presence of hydrocarbons and associated fluids in reservoir rocks (strata) below the surface of the earth. A determination is made of the relationship between the porosity and the water (salt water or fresh water) in the pore spaces of the reservoir rocks. When certain relationships exist between the porosity and the water, the presence of recoverable hydrocarbons and associated fluids and non-water miscible substances is indicated.

i 3 1 5 2 o -2 '-i7 OR 382O939O United States Patent 1191 1111 3,820,390 F orgotson June 28, 1974 METHOD OF RECOGNIZING THE 3,638,484 2/1972 Tixier 73/152 PRESENCE OF HYDROCARBONS AND ASSOCIATED LU IN RESERVOIR Primary Examiner-Jerry W, Myracle ROCKS BELOW THE SURFACE OF THE Attorney, Agent, or Firm-Smyth, Roston and'Pavitt EARTH [76] Inventor: James M. Forgotson, 147 Jordan St., [57] ABSTRACT Shreveport, La. 71101 p r This invention relates to methods of recognizing the [22] Flled' July 1970 presence of hydrocarbons and associated fluids in res- [21] Appl. No.: 58,400 ervolr rocks (strata) below the surface of the earth. A determination is made of the relationship between the porosity and the water (salt water or fresh water) in [52] US. Cl. 73/152 h k Wh 51 lm. c1 E2lb 49/00 the P i e reservo [58] Field of Search 73/152 151 re1at1onsh1ps exist between the porosxty and the water,

the presence of recoverable hydrocarbons and asso- 56] References Cited ciated fluids and non-water miscible substances is indit d. UNITED STATES PATENTS ca 6 3,180,141 4/1965 Alger 73/152 22 Claims, 11 Drawing Figures 0.0 10.0 zaa 30.0 40.0 5% 640 PAIENIEDJum 1914 3820.390 f SHEEISUFS' JITdA/YE y;

PAIENIEDJUI28 1914 I 3.820.390

sate: s (I 6 uan away METHOD OF RECOGNIZING THE PRESENCE OF HYDROCARBONS AND ASSOCIATED FLUIDS IN RESERVOIR ROCKS BELOW THE SURFACE OF THE EARTH This invention relates to enhanced methods of recognizing the presence of hydrocarbons, such as oil and gas and associated fluids, in reservoir rocks (strata) below the surface of the earth. The invention further relates to methods of using known data in novel ways to obtain these determinations. The invention is particularly adapted to facilitate the discovery of hydrocarbons such as oil and gas in locations where hydrocarbons have been unrecognized previously.

As hydrocarbons such as oil and gas are recovered from the earth and the earth becomes depleted of such hydrocarbons, it becomes increasingly difficult to find new areas where hydrocarbons are stored in the ground. For example, as the earth becomes depleted of hydrocarbons under land surfaces in moderate climes such as Texas, exploration has occurred in Alaska and also in the oceans off the shores of the Continental United States and elsewhere. Furthermore, both the ratio of successful wells to the number of wells drilled in each year and the number of successful completions are generally decreasing in progressive years.

As the exploration for oil and gas continues, the amount of information available correspondingly increases. For example, measurements are now made and various parameters are used which provide some value to assist in determining whether or not hydrocarbons such as oil and gas exist in a particular reservoir rock. Measurements are made of the electrical resistance (or electrical conductivity) of the reservoir rocks at particular depths to facilitate the interpretation of the contents of the reservoir rock. Such measurements are particularly made to aid in the determination of the percentage of water in the pores of the reservoir rocks. Measurements are also made of the transit time of a sonic wave through the rocks exposed to the borehole as a means of measuring the porosities of the reservoir rock exposed to the borehole. These measurements also facilitate an interpretation of the percentage of water saturation in the reservoir rocks at particular depths. Measurements are also made to determine the percentage of shale in the reservoir rocks at particular depths.

In spite of the vast amount of data which is collected to help in the discovery and recovery of hydrocarbons and associated fluids, the recognition of the presence of such hydrocarbons and associated fluids in the reservoir rocks is still quite imprecise. For example, in spite of the fact that various data has been heretofore obtained, the data has not been accurately correlated and properly used adequately to recognize the presence of hydrocarbons and associated fluids in the reservoir rocks. This has inhibited, on a truly scientific basis, the recognition of the presence of such hydrocarbons and associated fluids in the reservoir rocks. For example, since the presence of hydrocarbons and associated fluids has not been fully recognized in reservoir rocks at various depths below the surface of the earth wells which might otherwise have been productive have been abandoned because the presence of hydrocarbons and associated fluids has not been recognized as a result of failure to recognize the true relationships of the data obtained.

This invention provides a method of recognizing the presence of hydrocarbons such as oil, gas and associated fluids in each reservoir rock below the earths surface. The invention uses data already available or obtainable in obtaining the practice of such a method. For example, the invention provides a recognition of the presence of such hydrocarbons and associated fluids in the reservoir rock by the relationships between the porosities of the reservoir rocks and the percentage of water in the pore spaces of the reservoir rock. This recognition is made on the basis that all pore spaces not filled by water are filled by hydrocarbons or associated fluids or other substances which are not water miscible. The relationships between the porosities and the water in the pore spaces in the reservoir rocks have been used to recognize the presence of hydrocarbons in reservoir rocks.

The porosity determinations may be derived from any source, and numbers representing the measure of the source can be used direct as long as they bear a direct relationship to the percent of porosity in the reservoir rock. The percentage of water (salt water or fresh water) can be derived from any source, and numbers representing the measure of the source can be used direct as long as there is a direct proportion between these numbers and the percentage of water (salt water or fresh water) in the pore spaces of the reservoir rock.

minations of the sonic characteristics of the reservoir rocks have been combined with determinations of the electrical resistivity (or conductivity) of these rocks to provide an indication of the percentage of the water in the pore spaces of the rocks.

In one embodiment of the invention, plots have been made with the percentage of porosity of the reservoir rock along one co-ordinate axis and the percentage of water in the reservoir rock along another co-ordinate axis. The slope of this plot has provided a recognition of the presence or absence of producible hydrocarbons and associated fluids from the reservoir rocks. For example, when the plot shows that increases in the percentages of porosities in the reservoir rocks occur at the same observed points with decreases in percentages of water in the pore spaces of the reservoir rocks, then the reservoir rocks contain producible hydrocarbons and/or associated fluids.

The method constituting this invention has been used with good results in a number of different wells to recognize the presence of hydrocarbons and associated fluids in various reservoir rocks at random depths. The presence of these hydrocarbons and associated fluids has been proved by the recovery of substantial quantities of hydrocarbons and associated fluids at such random depths.

In the drawings:

FIGS. 1a, 1b and 1c illustrate graphs of the relationships between the porosities of the reservoir rocks and the percentages of water in the pore spaces of the reservoir rocks at corresponding depths;

FIG. 2 is a view schematically illustrating a well and one method used for determining the electrical resistivity of the reservoir rocks in the well at particular depths;

FIG. 3 is a view schematically illustrating a well and one method used for determining the sonic characteristics of the reservoir rocks in the well at the particular depths;

FIG. 4 is a chart illustrating one method of using the measurements obtained from the methods of FIGS. 2 and 3 to determine the percentage of water in the pore spaces of the reservoir rocks in the well at the particular depths;

FIG. 5 illustrates graphs of the electrical resistivity obtained from the method'shownin FIG. 3;

FIG. 5a illustrates graphs of the radioactivity of the reservoir rocks in the well at the particular depths and the spontaneous potential of the reservoir rocks in the well at the particular depths; and

FIGS. 6a, 6b and 6c illustrate schematically how hydrocarbons and other associated fluids are trapped in different ways in reservoir rocks.

In one embodiment of the method constituting this invention, the resistivity of the reservoir rocks in a well is determined by known techniques and known equipment. In FIG. 2, acable 10 and a sonde '11 are used in a well hole 12 defined by walls 14 of the reservoir rocks 16 surrounding the hole.

The determination of the resistivity of the reservoir rocks in the well is made by a continuous measurement of the impedance to the current flowing between various points in the well bore when a particular current is applied through the sonde 11 and through the rocks exposed in the well bore. These measurements are recorded by a galvanometer or other equipment on the surface of the earth and the plot of these measurements is commonly known as an electrical resistivity well log.

FIG. 5 illustrates as at 20 the measured resistivity of the rock exposed inthe well bore at progressive depths below the earths surface. The values of resistivity of the rock increase progressively toward the right in FIG. 5, as shown by the scale in FIG. 5. The resistivity of the earth is indicated in FIG. 5 for progressive depths between approximately 8,900 feet and 9,100 feet in a well designated as J. R. Pepper No. l in Blueford Bell Survey, Rusk County, Texas.

As will be appreciated, the resistivity of the rocks exposed in the well bore at progressive depths below the earths surface is in general dependent upon three factors. One factor is the mineralogical (lithological) character of the rocks (matrices) themselves. A second factor is the salinity (conductivity) of the water in the pore spaces of the rocks and the electrical characteristics of the substances other than water which are held in the pore spaces of the reservoir rock. A third factor is the amount of pore space filled with water in the rocks.

The percent of water in the pore spaces of the reservoir rocks is determined by a known and widely used formula in which the percentage of porosity of the reservoir rocks, the degree of salinity of the water in the reservoir rocks and the observed resistivity of the reservoir rocks are components. A method of measuring the porosity of the reservoir rocks is to measure the elapsed time of the sonic waves passing through the rocks in the well bore. Standard tables designate the percentage of porosity for various travel times through different lithological matrices. This sonic measurement is made by a device 30 (FIG. 3) disposed on an electrical cable 32 in the well bore. The device 30 generates sound waves at one position on the device and receives the sound waves at a second position on the device, the second position being separated by a known distance from the first position. The time required for the sound wave to travel on the sound wave patterns through the rocks between the first and second positions on the device 30 provides a measure of the porosity of the rocks between the first and second positions. These measurements are recorded as a continuous reading.

The indications provided by using the resistivity methods of FIG. 2 and the sonic measurements of FIG. 3 are combined by use of the chart shown in FIG. 4 to determine the percentage of salt water in the pore spaces of the reservoir rocks. In the chart shown in FIG. 4, the resistivity of the earth is indicated as R, and the sonic measurement of time is indicated along the abscissa as A,. In order to provide such a determination, the resistivity R,,, of the water in the area around the well has to be known or assumed. Furthermore, the velocityV of the sound in the earth around the well has to be known or assumed. The chart then indicates the relative percentage of the water in the pore spaces of the reservoir rocks by the oblique lines designated as 8,, and having numerical designations of 10, 20, 30, etc. The chart shown in FIG. 4 is a substantial duplicate of Chart D-l8 of the log interpretation charts published by the Schlumberger Well Surveying Corporation and copyrighted in 1962.

The use of the chart constituting FIG. 4 may be seen from a particular example. Assume that the resistivity R, of the reservoir rocks is measured as 12 ohms by the method shown in FIG. 2 and that the increment A, in time (as measured by the method shown in FIG. 3) for the reception of the sonic pulses is 70 seconds. Assume further that the resistivity R of the water in the area is measured, known or assumed as 0.02 ohm. Then R,/R l2/0.02 600. Assume further that the velocity of sound V is measured, known or assumed as 19,500. This value is indicated at the bottom of the chart shown in FIG. 4 and is projected horizontally as at 40 to a position 42 intersecting at the bottom of the chart an oblique line designated as 44 and representing A This point of intersection is then projected vertically as at 46 until it intersects a horizontal line 48 having a value of 600 and representing R,/ R As will be seen, the intersection occurs at a value of 5,, of approximately 30, which indicates the relative percent of water in the pore spaces of the reservoir rocks around the well at the depth undergoing analysis.

In using the chart shown in FIG. 4 to provide the methods constituting this invention, the actual values of such parameters as the velocity V,,, of sound and the resistivity R of water do not have to be known. One reason is that comparative values of percentages of water and of porosities bear a direct relationship at each depth to actual values of percentages of water and porosities at the same depth and are therefore acceptable for use in making the plot.

Neither the absolute measurement of the percentage of porosity nor the absolute measurement of the percent of salt water in the pore spaces of the reservoir rocks is necessary for the successful use of the methods constituting this invention. Any numbers can be used successfully if these numbers bear a direct relationship to the true percentage of porosity of the reservoir rocks. Similarly, any numbers can be used successfully if they bear a direct relationship to the true percentage of salt water in the pore spaces of the reservoir rocks.

For example, the direct resistivity reading may be plotted along a first co-ordinate axis and the travel time of the sonic waves may be plotted along a second coordinate axis. The same result is obtained hereby as though the percentage of water in the pore spaces of the reservoir rocks is plotted along the first co-ordinate axis and the porosity of the reservoir rocks is plotted along the second coordinate axis. This becomes important because in most instances the salinity (R,,.) of the water in the pore spaces of the reservoir rocks is not known.

The accuracy of the determinations made as to the presence or absence of producible water from the res ervoir rock is dependent upon the measurements made either in the field in a downhole survey or in a laboratory test from samples taken from the well bore. The in situ measurements are considered superior because of the direct inferences made from the downhole survey without distortion which occurs from the beginning of the drilling or coring of the sample through the end of the laboratory analysis.

Since the matrices of reservoir rocks usually constitute lithological mixtures, the porosity measurements are not absolute. Various methods exist for refining such porosity measurements. For example, many sandstone reservoirs and limestone reservoirs are contaminated with shale. The presence of this shale causes the measurements of the porosity of the reservoir rocks to be higher than the true values. Various techniques exist to compensate for the contamination caused by the shale in the measurements of porosity. By these meth ods, an attempt is made to determine the true effective porosity of the reservoir rocks. When available, gamma ray surveys made either in the open hole or in the cased hole can be used to provide such compensation. These gamma ray surveys are known and widely used. In order to decrease the porosity measurements to their proper values, a factor obtained from the gamma ray survey is used to increase to a proper value the observed velocity of the sound waves through the contaminated matrix.

It has been determined that an increase in gamma ray response is indicative of an increase in shale content. FIG. 5a illustrates as at 50 variations in the intensity of the gamma ray reception at different depths in a well bore and therefore is an indication of the shale present at such depths. Specifically, curve 50 in FIG. 5a represents the intensity of the gamma ray reception for depths between approximately 8,900 feet and 9,100 feet below the earths surface in the well designated as J. R. Pepper No. l in Blueford Bell Survey, Rush County, Texas. High values of gamma ray reception are toward the right in FIG. 5a and indicate increases in shale content.

The measurement of spontaneous (self) potential is also known and widely used as an indication of porosities in a reservoir. The current indicated by the plot of the self potential curve is generated in the rocks of the well bore by the mixing (invasion) in the rocks of the well bore of the fluid in the well bore with the fluids in the exposed rocks, the fluids in the exposed rocks having a different salinity than the fluid in the well bore.

FIG. 5a also shows as at 60 the spontaneous (self) potential which is often considered a measure, although not a direct measure, of porosity. High values of self potential are toward the right in FIG. 5a and indicate increases in shale content.

FIG. 3 shows a caliper device 62 on the sonic tool for measuring the diameter of the well bore at all points. This caliper device may be used on any other tool as the occasion demands. Variations in the diameter of the well bore affect the spontaneous potential response. Variations in the differential salinities between the well bore fluid and the fluid in the exposed rocks also affect the spontaneous potential response.

The readings of the spontaneous potential response, the computed salinity of the water in the reservoir rock and the variations in the diameter of the bore hole in the reservoir rock are factors in a formula which is well known and generally accepted to reduce, to a value representing the effective porosity, the observed porosity caused by the presence of shale in the reservoir rocks. This method of correcting for shale is often not as accurate as the method employing gamma rays to correct for shale.

The suite of logs to practice the methods constituting this invention should include logs for indicating the porosity of the reservoir rocks and the resistivity of the reservoir rocks. This suite is examined to determine the gross intervals of the logs to be studied in detail. In this instance, the gross interval constitutes that portion of the log which skilled personnel such as geologists, log analysts or well operators wish to study by the methods constituting this invention to determine whether recoverable hydrocarbons and associated fluids are present. The suite of logs is then digitized on an incremental basis,preferably on a basis of one-foot increments.

The results of the digitized logs, namely, the porosity of the reservoir rocks and the resistivity of the reservoir rocks, can then be plotted directly. Alternatively, the percentage of porosity of the reservoir rocks and the percentage of the salt water in the pore spaces of the reservoir rocks can be computed by generally accepted and universally used methods and these values can be plotted. The time for these is shortened by using computerized methods and a print-out received showing the following essentials in tabulated form:

a. Depth;

b. Porosity computed from observed velocity of sound waves (other sources may be used);

c. Eflective porosity after computing and considering effect of shale;

d. (Computed porosity effective porosity) divided by computed porosity percent of shale in pore spaces;

f. Percentage of salt water in pore spaces.

In practice, the tabulated print-outs are studied by skilled personnel such as geologists, log analysts and well operators. Plots are made of those reservoir rocks considered to have sufficient thickness (from top to bottom of the reservoir rocks) and sufficient porosities to be considered commercially productive of hydrocarbons and associated fluids. These plots can be made manually or by a mechanical plotter. The plots in these instances show the relationship of the percentage of porosity of the reservoir rocks and the percentage of water in the pore spaces of the reservoir rocks.

Actual relationships between the percentages of water in the pore spaces of the reservoir rocks and effective porosities of the reservoir rocks are plotted in FIGS. la, 1b and 10 for the J. R. Pepper No. 1 well at different depth levels. In these Figures, the percentages of water in the pore spaces of the reservoir rocks are plotted along the abscissa and the percentages of the effective porosities of the reservoir rocks are plotted along the ordinate. As will be described subsequently in detail, the slope of the plot in FIGS. la, 1b and it determines with some accuracy the presence or absence at the different depths of hydrocarbons such as oil and gas and associated fluids.

In providing an understanding of the plots shown in FIGS. 1a, 1b and 1c, certain natural phenomena should probably be first explained. For example, any given volume of any given water-wet reservoir rock contains a certain volume of non-producible water. This volume of non-producible water results from several factors. One of these factors constitutes adhesive forces which are determined by the lithological character of the rock matrix and the diameter of the individual pore spaces in the rock matrix. Another factor constitutes the cohesive forces within the water itself, these cohesive forces controlling the droplet size. The cohesive forces are affected by such factors as the composition and concen tration of the solutes in the water.

The quantity of non-producible water contained in the rocks is not affected by the percentage of pore spaces in the rocks. In other words, there is no relationship between the percentage of the porosity of the rocks and the quantity of non-producible water contained within the rocks. Furthermore, an increase of porosity in the rocks will result in a reduced percentage of pore space occupied by the non-producible water. In other words, since the amount of non-producible water in the rocks is independent of the percentage of porosity of the rocks, increased porosities of the rocks will cause the percentage of the non-producible water in the rocks to decrease.

Under certain circumstances, when the percentage of the water in the pore spaces of the rocks decreases at the same depth that the porosity of the rocks increases, the presence of hydrocarbons and associated fluids is indicated. This is because the hydrocarbons are able to occupy the space not occupied in the pores of the rocks by the water.

FIGS. 6a, 6b and 6c are schematic cross sections of reservoir rocks and show some of the various types of hydrocarbon traps wherein the segregation of hydrocarbons from producible water occurs because of differences in the specific gravities of different fluids. FIG. 6a schematically illustrates the accumulation of hydrocarbons 70 at the top of an anticline or dome. FIG. 6b schematically illustrates the accumulation of hydrocarbons 72 at the top of a lenticular reservoir rock. FIG. 6c schematically illustrates the accumulation of hydrocarbons 74 in a trap caused by faulting.

As will be seen in FIG. la, plots are provided of the percentage of water versus porosities at 1-foot intervals in a reservoir rock in the J. R. Pepper No. 1 well from depths of 8,560 feet to 8,587 feet. The different numbers in F IG. 1 represent successive intervals of one foot below the level of 8,560 feet.

Lines are then drawn through the positions representing progressive depths on the graph. For example, a line (or segment) is drawn through the point positions designated by the numerals 7, 6, 5, 4 and 3 in FIG. 1a. This line is indicated at segment 90 in FIG. 1a. As will be seen, the line segment 90 slopes upwardly and to the left. This slope upwardly and to the left indicates that hydrocarbons and associated fluids are present at depths between 8,563 and 8,567 feet in the J. R. Pepper No. 1 well.

As will be seen by a segment 92 having depths designated by the numerals 8, 9 and 10, a transition zone (from hydrocarbons and associated fluid to producible water) exists in the reservoir rock between depths of 8,568 and 8,570 feet. This zone contains hydrocarbons and associated fluids together with variable quantities of water. The mixture of variable quantities of hydro carbons and associated fluids and variable quantities of water in the zone indicates that attempts should not be made to recover hydrocarbons and associated fluids at these depths.

The plot of percentage of water and percentages of porosities in the zone between 8,570 feet and 8,577 feet is indicated at segment 94 in FIG. 1a. Since the plot slopes upwardly and to the right, this indicates that the percentage of water increases as the porosity increases. This indicates that this zone of the reservoir rock is practically saturated with water between the depths of 8,570 and 8,5 77 feet. Since a segment 96 between depths of 8,578 and 8,580 feet also slopes upwardly and to the right, this zone of the reservoir rock is also practically saturated with water. This indicates that attempts should not be made to recover hydrocarbons and associated fluids at depths between 8,570 and 8,577 feet and depths between 8,578 and 8,580 feet.

FIG. lb represents plots of percentage of water versus porosities between depths of 8,615 to 8,636 feet. As will be seen from the upward and leftward slopes of segment 100, the relative effective porosity increases as the relative percentage of water saturation decreases for depths between 8,634 and 8,637 feet. This indicates that hydrocarbons and associated fluids are present at depths between 8,634 and 8,637 feet. Hydrocarbons and associated fluids also exist in the zone between 8,627 and 8,632 feet, as indicated by the slope of a segment 102 in FIG. lb. Hydrocarbons and associated fluids likewise exist in the zone between 8,618 and 8,625 feet, as may be seen from the slope of a segment 104 in FIG. 1b.

FIG. 10 provides additional plots of percentages of water versus porosities at various depths in J. R. Pepper No. 1 well. These plots occur between depths of 7,208 feet and 7,229 feet. As will be seen, a segment is provided of plots between 7,220 and 7,229 feet. Since this plot slopes upwardly and to the left, the presence of hydrocarbons and associated fluids is indicated. As will be seen, the progress of the plot of the segment must be in sequence but does not have to occur for depths in consecutive order.

FIG. 10 further illustrates two other plots of zones which occur between 7,208 and 7,229 feet. These plots are respectively designated as 112 and 114. In both of these plots, the slope of percentages of water versus porosity is upwardly and to the left. This indicates the presence of hydrocarbons and associated fluids at depths between 7,214 and 7,216 feet and between 7,208 and 7,212 feet.

When the percentage of water in the pore spaces of any reservoir rock approaches a maximum of thirty percentage (30 percent), hydrocarbons and associated fluids generally exist in the reservoir rocks whether or not the plotted curves of percentage of water versus porosity slope to the left or in any other direction. Apparently, with such low values for relative percentage of water saturation, sufficient space exists in the rock formation so that hydrocarbons and associated fluids exist in the reservoirs in the rocks regardless of the slope of the curve of percentage of water versus porosity.

In order to have hydrocarbons and associated fluids in a given zone, the plot of percentage of water versus porosity, such as shown in FIGS. 1a, 1b and 10, should slope to the left from the vertical at least 3-5 to compensate for probable deficiencies in the precision of any logging system. A slope to the left of less than 3-5 to the vertical or to the right generally indicates almost complete water saturation.

A horizontal plot of percentage of water versus porosity is considered as a plot to the right and therefore indicative of practically complete water saturation. Any plot having a minimum of 30 percent of water and sloping to the left at an angle of or less from the horizontal should be considered as indicative of practically complete water saturation of the reservoir rock. This indicates an abrupt change in the size of the pore spaces of the reservoir rock.

Usable permeability is indicated when the plot defines a continuous segment constituting a straight line and each new segment indicates a break in the vertical permeability. Disconnections between two or more segments in any one reservoir rock is indicative of one or more depths at which permeability is practically non-existent. Each segment should be a straight line and the closer each point falls to the mean, the better is the permeability. A plot composed of random points from which no multiple-point segment can be drawn indicates the lack of permeability in the reservoir rock. Primarily there is some relationship, although different in each instance, between porosity and permeability. Permeability is the factor which allows the movement of fluids from one point to another in a reservoir rock. The freedom of movement of the fluid is a measure of the permeability.

Although this application has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

We claim:

1. A method of determining the presence of hydrocarbons and associated fluids at given depths in a reservoir rock, including the following steps:

determining the percentage of water at the given depths in the reservoir rock,

determining the percentage of porosity at the given depths in the reservoir rock, plotting the percentage of water along one coordinate axis and the percentage of porosity at the given depths in the reservoir rocks along a second coordinate axis, 7

selecting a particular coordinate quadrant for the determination of plots of the percentage of water along one coordinate axis and the percentage of porosity at the given depths in the reservoir rocks along the second coordinate axis, and

selecting zones in which a particular coordinate relationship in the particular coordinate quadrant exists between the plot of the percentage of water at the given depths in the reservoir rock and the plot of the percentage of porosity at the given depths in 6 the reservoir rock, the particular quadrant relationship providing a percentage of water less than a particular value in the pore spaces of the reservoir rock, the particular coordinate relationship being represented by simultaneous increases in the percentage of porosity and decreases in the percentage of water at the given depths.

2. The method set forth in claim 1 wherein the percentage of porosity of the reservoir rock at the successive depths is determined by obtaining an observed porosity of such given depths and then compensating this observed porosity for the effect of shale in the reservoir rock.

3. The method set forth in claim 2 wherein the relationship between the effective porosity of the reservoir rock and the percentage of water in the reservoir rock at the given depths is plotted along coordinate axes and wherein the presence of hydrocarbons and associated fluids in the reservoir rock at the given depths is indicated by plots in the particular quadrant when the effective porosity of the reservoir rock increases and the percentage of water in the reservoir rock decreases at a particular angle from the axis of the porosity at each given depth.

4. A method as set forth in claim 2 wherein the values of the porosity are plotted along the ordinate of a graph and the percentage of water in the reservoir rock are plotted along the abscissa and where such plot in the particular quadrant has a slope of at least a first particular angle from the ordinate in the particular coordi nate quadrant where the first particular angle is dependent upon the scales of values along the abscissa and the ordinate.

5. A method as set forth in claim 4 wherein the slope of the plot of the values of porosity and the percentage of water in the reservoir rock in the particular quadrant is at least a first particular angle from the abscissa in the particular coordinate quadrant where the first particular angle is dependent upon the scales of values along the abscissa and the ordinate.

6. The method set forth in claim 1 wherein the percentage of water at the given depths in the reservoir rock is determined by obtaining. an observed resistivity of the reservoir rock at the given depths and the sonic transit time through the reservoir rock at the given depths and by determining the relationship between the observed resistivity and the sonic transit time in the reservoir rock at the given depths.

7. A method of determining the presence of hydrocarbons and associated fluids at given depths in a reservoir rock, including the steps of:

determining the resistivity at the given depths in the reservoir rock,

detennining the porosity of the reservoir rock at the given depths,

plotting along one coordinate axis the resistivity at the given depths in the rock and plotting along a second coordinate axis the porosity of the reservoir rock at the given depths,

selecting a particular quadrant for the determination of the plots of the resistivity along the first coordinate axis and the porosity of the reservoir rock along the second coordinate axis, and determining the presence of hydrocarbons and associated gases at the given depths in the reservoir rock in accordance with plots in the particular quadrant representing increases in porosity at different depths and simultaneous increases in resistivity at such different depths.

8. The method set forth in claim 7 wherein the presence of hydrocarbons and associated fluids at the given depths in the reservoir rock is determined by simultaneous increases in resistivity and increases in porosity at a particular angle in the reservoir rock at the given depths.

9. The method set forth in claim 8 wherein the plot of porosity is along the ordinate and the plot of resistivity is along the abscissa and the presence of hydrocarbons and associated gases is determined in the particular quadrant for slopes greater than from the ordinate and at least a second particular angle from the abscissa where the first and second particular angles are dependent from the scales of values along the abscissa and the ordinate.

10. A method of determining the presence of hydrocarbons and associated fluids at given depths in a reservoir rock, including the steps of:

determining the resistivity of the reservoir rock at the given depths,

determining the porosity of the reservoir rock at the given depths,

determining the coordinate relationships between the resistivity of the reservoir rock at each given depth and the porosity of the reservoir rock at each such given depth,

selecting a particular quadrant for the determination of the resistivity of the reservoir rock at each given depth and the porosity of the reservoir rock at such given depth, and

selecting the depths at which the hydrocarbons and the associated fluids are present in accordance with the occurrence in the particular coordinate quadrant of an increase in the resistivity of the reservoir rock and an increase in the porosity of the reservoir rock at the given depths.

11. The method set forth in claim 10 wherein a plot is made along two coordinate axes of the coordinate relationships between the resistivity of the reservoir rock at each given depth and the porosity of the reservoir rock at each such given depth and wherein the given depths are selected in which the plot constitutes a segment having a particular slope in the particular coordinate quadrant of at least a particular angle from the axis representing the porosity of the reservoir rock where the particular angle is dependent upon the scales of values along the two coordinate axes.

12. The method set forth in claim 11 wherein the given depths are selected where the slope of the plotted segment has at least a particular angle from the axis representing the resistivity of the porous rock in the particular coordinate quadrant where the particular angle is dependent upon the scales of values along the abscissa and the ordinate.

13. The method set forth in claim 12 wherein the observed porosity of the reservoir rock at the given depths is determined by sonic methods, the percentage of shale in the pore spaces of the reservoir rock at the given depths is determined by methods using gamma ray, and the electrical resistivity of the reservoir rock at the given depths is determined by methods using electrical currents.

14. A method of determining the presence of hydrocarbons and associated fluids at given depths in a reservoir rock, including the steps of:

determining the porosity of the reservoir rock at the given depths whether expressed in percent or by numbers bearing a direct relationship to the porosity at each given depth, determining the water in the reservoir rock at the given depths whether expressed in percent or by numbers bearing a direct relationship to the percentage of water at each given depth,

determining the coordinate relationship between the porosity, along a first axis, of the reservoir rock at each given depth and the water, along a second axis coordinate with the first axis, in the reservoir rock at each given depth, and

determining given depths for the presence of hydrocarbons and associated fluids when the porosity of the reservoir rock increases along the first axis in a particular coordinate quadrant at such given depths and the percentage of water decreases along the second axis in the particular coordinate quadrant at such given depths.

15. The method set forth in claim 14 wherein the presence of hydrocarbons and associated fluids is determined at given depths when the decrease, along the first axis in the particular coordinate quadrant, in the percentage of water in the reservoir rock relative to the increase, along thesecond axis in the particular coordinate quadrant, in the porosity of the reservoir rock is at an angle of at least a particular magnitude relative to the direction representing increases in the porosity of the reservoir rock.

16. The method set forth in claim 15 whereinthe relationship between the porosity of the reservoir rock at the given depths and the water in the reservoir rock at the given depths is determined by plotting the porosity along one of two co-ordinate axes and plotting the water along the other of the two co-ordinate axes.

17. The method set forth in claim 15 wherein the porosity of the reservoir rock at the given depths is determined by determining the observed porosity of the reservoir rock at such given depths and the pore spaces occupied by shale in the reservoir rock at such given depths and by compensating the observed porosity for the pore spaces occupied by shale in the reservoir rock.

18. The method set forth in claim 15 wherein the water in the reservoir rock is determined by determining the resistivity of the reservoir rock at such given depths, the transit time of sonic pulses in the reservoir rock at such given depths, the resistivity of salt water at such depths and the observed porosity of the reservoir rock at such given depths.

19. The method set forth in claim 15 wherein the presence of hydrocarbons and associated fluids is indicated at given depths when the decrease in the percentage of water in the reservoir rock relative to the increase in the porosity of the reservoir rock is at least at a particular angle relative to the direction representing decreases in the percentage of water in the reservoir rock in the particular coordinate quadrant where the particular angle is dependent upon the scales of values along the abscissa and the ordinate.

20. A method of determining the presence of hydrocarbons and associated fluids at given depths in a reservoir rock, including the following steps:

determining the percentage of water at the given depths in the reservoir rock,

determining the percentage of porosity at the given depths in the reservoir rock,

determining the coordinate relationship between the percentage of water and the percentage of porosity at the given depths in the reservoir rocks,

selecting a particular coordinate quadrant for the determination of the relationship between the percentage of water at the given depths in the reservoir rock and the percentage of porosity at the given depths in the reservoir rock, and

selecting zones in which a particular relationship ex ists in the particular coordinate quadrant between the percentage of water at the given depths in the reservoir rock and the percentage of porosity at the given depths in the reservoir rock, the particular relationship occurring when a percentage of at least approximately 30 percent of water exists in the pore spaces of the reservoir rock and the porosity of the reservoir rock increases at different the abscissa and the ordinate.

22. The method set forth in claim 21 wherein the percentage of water at the given depths in the reservoir rock is determined by obtaining an observed resistivity of the reservoir rock at the given depths and the sonic transit time through the reservoir rock at the given depths and by determining the relationship between the observed resistivity and the sonic transit time in the res ervoir rock at the given depths. 

1. A method of determining the presence of hydrocarbons and associated fluids at given depths in a reservoir rock, including the following steps: determining the percentage of water at the given depths in the reservoir rock, determining the percentage of porosity at the given depths in the reservoir rock, plotting the percentage of waTer along one coordinate axis and the percentage of porosity at the given depths in the reservoir rocks along a second coordinate axis, selecting a particular coordinate quadrant for the determination of plots of the percentage of water along one coordinate axis and the percentage of porosity at the given depths in the reservoir rocks along the second coordinate axis, and selecting zones in which a particular coordinate relationship in the particular coordinate quadrant exists between the plot of the percentage of water at the given depths in the reservoir rock and the plot of the percentage of porosity at the given depths in the reservoir rock, the particular quadrant relationship providing a percentage of water less than a particular value in the pore spaces of the reservoir rock, the particular coordinate relationship being represented by simultaneous increases in the percentage of porosity and decreases in the percentage of water at the given depths.
 2. The method set forth in claim 1 wherein the percentage of porosity of the reservoir rock at the successive depths is determined by obtaining an observed porosity of such given depths and then compensating this observed porosity for the effect of shale in the reservoir rock.
 3. The method set forth in claim 2 wherein the relationship between the effective porosity of the reservoir rock and the percentage of water in the reservoir rock at the given depths is plotted along coordinate axes and wherein the presence of hydrocarbons and associated fluids in the reservoir rock at the given depths is indicated by plots in the particular quadrant when the effective porosity of the reservoir rock increases and the percentage of water in the reservoir rock decreases at a particular angle from the axis of the porosity at each given depth.
 4. A method as set forth in claim 2 wherein the values of the porosity are plotted along the ordinate of a graph and the percentage of water in the reservoir rock are plotted along the abscissa and where such plot in the particular quadrant has a slope of at least a first particular angle from the ordinate in the particular coordinate quadrant where the first particular angle is dependent upon the scales of values along the abscissa and the ordinate.
 5. A method as set forth in claim 4 wherein the slope of the plot of the values of porosity and the percentage of water in the reservoir rock in the particular quadrant is at least a first particular angle from the abscissa in the particular coordinate quadrant where the first particular angle is dependent upon the scales of values along the abscissa and the ordinate.
 6. The method set forth in claim 1 wherein the percentage of water at the given depths in the reservoir rock is determined by obtaining an observed resistivity of the reservoir rock at the given depths and the sonic transit time through the reservoir rock at the given depths and by determining the relationship between the observed resistivity and the sonic transit time in the reservoir rock at the given depths.
 7. A method of determining the presence of hydrocarbons and associated fluids at given depths in a reservoir rock, including the steps of: determining the resistivity at the given depths in the reservoir rock, determining the porosity of the reservoir rock at the given depths, plotting along one coordinate axis the resistivity at the given depths in the rock and plotting along a second coordinate axis the porosity of the reservoir rock at the given depths, selecting a particular quadrant for the determination of the plots of the resistivity along the first coordinate axis and the porosity of the reservoir rock along the second coordinate axis, and determining the presence of hydrocarbons and associated gases at the given depths in the reservoir rock in accordance with plots in the particular quadrant representing increases in porosity at different depths and simultaneous increases in resistivity at such different depths.
 8. The method set forth in claim 7 wherein the presence of hydrocarbons and associated fluids at the given depths in the reservoir rock is determined by simultaneous increases in resistivity and increases in porosity at a particular angle in the reservoir rock at the given depths.
 9. The method set forth in claim 8 wherein the plot of porosity is along the ordinate and the plot of resistivity is along the abscissa and the presence of hydrocarbons and associated gases is determined in the particular quadrant for slopes greater than from the ordinate and at least a second particular angle from the abscissa where the first and second particular angles are dependent from the scales of values along the abscissa and the ordinate.
 10. A method of determining the presence of hydrocarbons and associated fluids at given depths in a reservoir rock, including the steps of: determining the resistivity of the reservoir rock at the given depths, determining the porosity of the reservoir rock at the given depths, determining the coordinate relationships between the resistivity of the reservoir rock at each given depth and the porosity of the reservoir rock at each such given depth, selecting a particular quadrant for the determination of the resistivity of the reservoir rock at each given depth and the porosity of the reservoir rock at such given depth, and selecting the depths at which the hydrocarbons and the associated fluids are present in accordance with the occurrence in the particular coordinate quadrant of an increase in the resistivity of the reservoir rock and an increase in the porosity of the reservoir rock at the given depths.
 11. The method set forth in claim 10 wherein a plot is made along two coordinate axes of the coordinate relationships between the resistivity of the reservoir rock at each given depth and the porosity of the reservoir rock at each such given depth and wherein the given depths are selected in which the plot constitutes a segment having a particular slope in the particular coordinate quadrant of at least a particular angle from the axis representing the porosity of the reservoir rock where the particular angle is dependent upon the scales of values along the two coordinate axes.
 12. The method set forth in claim 11 wherein the given depths are selected where the slope of the plotted segment has at least a particular angle from the axis representing the resistivity of the porous rock in the particular coordinate quadrant where the particular angle is dependent upon the scales of values along the abscissa and the ordinate.
 13. The method set forth in claim 12 wherein the observed porosity of the reservoir rock at the given depths is determined by sonic methods, the percentage of shale in the pore spaces of the reservoir rock at the given depths is determined by methods using gamma ray, and the electrical resistivity of the reservoir rock at the given depths is determined by methods using electrical currents.
 14. A method of determining the presence of hydrocarbons and associated fluids at given depths in a reservoir rock, including the steps of: determining the porosity of the reservoir rock at the given depths whether expressed in percent or by numbers bearing a direct relationship to the porosity at each given depth, determining the water in the reservoir rock at the given depths whether expressed in percent or by numbers bearing a direct relationship to the percentage of water at each given depth, determining the coordinate relationship between the porosity, along a first axis, of the reservoir rock at each given depth and the water, along a second axis coordinate with the first axis, in the reservoir rock at each given depth, and determining given depths for the presence of hydrocarbons and associated fluids when the porosity of the reservoir rock increases along the first axis in a particular coordinate quadrant at such given depths and the percentage of water decreases along the sEcond axis in the particular coordinate quadrant at such given depths.
 15. The method set forth in claim 14 wherein the presence of hydrocarbons and associated fluids is determined at given depths when the decrease, along the first axis in the particular coordinate quadrant, in the percentage of water in the reservoir rock relative to the increase, along the second axis in the particular coordinate quadrant, in the porosity of the reservoir rock is at an angle of at least a particular magnitude relative to the direction representing increases in the porosity of the reservoir rock.
 16. The method set forth in claim 15 wherein the relationship between the porosity of the reservoir rock at the given depths and the water in the reservoir rock at the given depths is determined by plotting the porosity along one of two co-ordinate axes and plotting the water along the other of the two co-ordinate axes.
 17. The method set forth in claim 15 wherein the porosity of the reservoir rock at the given depths is determined by determining the observed porosity of the reservoir rock at such given depths and the pore spaces occupied by shale in the reservoir rock at such given depths and by compensating the observed porosity for the pore spaces occupied by shale in the reservoir rock.
 18. The method set forth in claim 15 wherein the water in the reservoir rock is determined by determining the resistivity of the reservoir rock at such given depths, the transit time of sonic pulses in the reservoir rock at such given depths, the resistivity of salt water at such depths and the observed porosity of the reservoir rock at such given depths.
 19. The method set forth in claim 15 wherein the presence of hydrocarbons and associated fluids is indicated at given depths when the decrease in the percentage of water in the reservoir rock relative to the increase in the porosity of the reservoir rock is at least at a particular angle relative to the direction representing decreases in the percentage of water in the reservoir rock in the particular coordinate quadrant where the particular angle is dependent upon the scales of values along the abscissa and the ordinate.
 20. A method of determining the presence of hydrocarbons and associated fluids at given depths in a reservoir rock, including the following steps: determining the percentage of water at the given depths in the reservoir rock, determining the percentage of porosity at the given depths in the reservoir rock, determining the coordinate relationship between the percentage of water and the percentage of porosity at the given depths in the reservoir rocks, selecting a particular coordinate quadrant for the determination of the relationship between the percentage of water at the given depths in the reservoir rock and the percentage of porosity at the given depths in the reservoir rock, and selecting zones in which a particular relationship exists in the particular coordinate quadrant between the percentage of water at the given depths in the reservoir rock and the percentage of porosity at the given depths in the reservoir rock, the particular relationship occurring when a percentage of at least approximately 30 percent of water exists in the pore spaces of the reservoir rock and the porosity of the reservoir rock increases at different depths and the percentage of water in the reservoir rocks decreases at such different depths.
 21. The method set forth in claim 20 wherein the angle of a plot of percentage of water in the reservoir rock at the given depths as the abscissa and the porosity of the reservoir rock as the ordinate in the particular coordinate quadrant is at least a first particular angle from the ordinate and at least a second particular angle from the abscissa where the first and second particular angles are dependent upon the scales of values along the abscissa and the ordinate.
 22. The method set forth in claim 21 wherein the percentage of water at the given depths iN the reservoir rock is determined by obtaining an observed resistivity of the reservoir rock at the given depths and the sonic transit time through the reservoir rock at the given depths and by determining the relationship between the observed resistivity and the sonic transit time in the reservoir rock at the given depths. 